1. Field of the Invention
The present invention relates to an apparatus and method for inspecting threaded objects and, specifically, for inspecting the externally threaded surface of an object such as a pipe.
2. Decription of the Prior Art
There is a need in many industries for an apparatus and method for measuring the thread dimensions of several different elements of a threaded surface, such as thread height, thread lead, thread taper, pitch diameter, chamfer dimensions, as well as discontinuities and defects in the thread surface to a high degree of accuracy.
The critical gaging of these thread elements is necessary due to the critical nature of many applications in which the threaded object is used. For instance, in oil and gas wells, the weight of a string of well pipe may exceed 500,000 pounds. The threaded pipe couplings must support this load and a thread failure can be extremely costly in money, as well as many times in the loss of human life. As a result, the American Petroleum Institute has published specifications which oil and gas well casing and tubing must meet. Other critical applications for thread inspection include the nuclear industry where threaded surfaces on nuclear rods must be inspected with a high degree of accuracy.
Electronic gages have been developed in recent years for use in inspecting the threaded surfaces of pipe. An example of such a gage can be found in U.S. Pat. No. 4,324,049 to Blose, issued Apr. 13, 1982, entitled "Gaging System and Method". Such gages typically employ contact feelers which contact the threaded surface to be measured at set points and mechanically measure the tolerances. One disadvantage of such devices is that the contact feelers can become worn with usage or become out of adjustment. Also, because the gage only takes a reading when the feeler is contacting the pipe, 100% coverage of the threaded surface during the inspection is not possible.
U.S. Pat. No. 4,315,688, to Pryor, issued Feb. 16, 1982, entitld "Electro-optical Sensor Systems For Thread and Hole Inspection", shows an apparatus for inspecting threaded objects, such as bolts, moving on a conveyor line past the inspection station. A detector having a light sensitive member picks up reflected light and produces analog output which is used to determine the quality of the threads. The light sensitive member has a light sensitive area sufficiently small to resolve the individual threads of the threaded object, to check to see that the correct number of threads are present.
It is also known to use video technology in the packaging and container industries. For instance, U.S. Pat. No. 4,435,641, issued Mar. 6, 1984, to Hajime, shows an inspection apparatus used to examine a bottle mouth as well as the bottle bottom simultaneously. The apparatus is not used to inspect externally threaded surfaces.
Inspection devices which use video cameras to inspect the interior of pipe by remotely reproducing a video image of the pipe interior are also known. U.S. Pat. No. 4,255,762, issued Mar. 10, 1981, to Takeyasu, et. al., shows such a device which is inserted into the interior of the pipe being inspected. The device is not revolved about the pipe exterior to inspect externally threaded surfaces.
Applicant is not aware of any prior art devices used to inspect externally threaded surfaces, such as pipe threads, in which a video camera is rotated about the stationary pipe to produce video signals, and in which the video signals are digitized and analyzed to determine the thread characteristics and to locate defects and abnormalities in the threaded surfaces.